This invention relates to scintillation cameras, commonly called gamma cameras, and is particularly concerned with correcting for nonuniformities or lack of positional correspondence between the distribution of radiation events in incremental areas of the camera field and in the displayed image.
In nuclear medicine, gamma camera systems are used to detect gamma ray photons emitted from a body in which a radioisotope has been infused. Scintillations occur where photons are absorbed by crystalline material. A typical system is based on the camera of Anger as disclosed in U.S. Pat. No. 3,011,057. The Anger camera, as does the camera in this disclosure, comprises an array of photosensitive devices such as photomultiplier tubes, usually hexagonally arranged, having their input ends adjacent a light conducting plate or disc. Beneath the disc is a scintillation crystal which converts incoming gamma photons into light photons or scintillations. A collimator is interposed between the scintillator and the emitting body so that photons emitted will impinge substantially perpendicularly to the planar scintillation crystal.
The array of photomultiplier tubes view overlapping areas of the scintillation crystal. The tubes produces a pulse for each scintillation event. Well-known electronic circuits are used to produce signals representing the x and y coordinates of the scintillations. A pulse height analyzer determines if the pulses are within amplitude limits and, if they are, a z signal is produced which controls a cathode ray oscilloscope display to produce a point of light on its screen at x and y coordinates corresponding with those of the scintillation event intercepted by the camera. A photographic film may be used as an image intergrator of the large number of light spots appearing on the screen of the cathode ray tube. A substantial number of events is required to make up the final picture of radioisotope distribution in the body tissue.
A problem in existing scintillation camera systems is that if a standard source having uniform isotope distribution is placed close to the crystal and a photograph is made of the image on the display tube, the photograph will show nonuniformity which results from so called "hot spots" under each photomultiplier tube and "cold spots" between the tubes. The transitions between hot and cold areas are rather gradual than abrupt. In other words, a spot or scintillation event actually occurring between photomultiplier tubes is sensed as being partially shifted under the tubes, causing a decrease in spot density between the tubes and an apparent increase in spot density under the tubes.
One method of correcting for nonuniformity uses a computer memory to store the count distribution taken from a flood field or uniform standard source. Some of the locations in the computer memory will then have fewer counts than they should have for a uniform radiation field. Hence, after the image is formed, the computer determines the areas of nonuniformity and corrects those areas by adding or subtracting counts and the image is displayed on a cathode ray tube. The problem with this method is that correction is made after the image of the body has been produced as opposed to the correction being made in real time as the image is being produced. This precludes correcting under dynamic conditions.
Another recently developed method is to use a computer based on a microprocessor. This method is believed to use the microprocessor for determining the deficiency of counts as compared with counts taken from a flat flood field and stored in a memory. The deficiencies are compared with randomly generated numbers and all of those memory locations which have counts in excess of a minimum result in inhibition of unblanking of the cathode ray display which amounts to subtracting counts in order to make the image uniform as opposed to injecting counts where there are deficiencies.